1. Field of the Invention
The present invention relates to graphene or a stack of graphene which has high conductivity and is permeable to ions of lithium or the like. Such graphene or stack of graphene is used for a material for a lithium-ion secondary battery, or the like.
Note that in this specification and the like, graphene refers to a one-atom-thick sheet of carbon molecules having sp2 bonds, and a stack of graphene refers to a structure in which a plurality of one-atom-thick sheets (the number of sheets is greater than or equal to 2 or less than or equal to 100, preferably greater than or equal to 2 or less than or equal to 50) of carbon molecules having sp2 bonds are stacked.
2. Description of the Related Art
Owing to excellent electric characteristics such as high conductivity or high mobility and excellent physical characteristics such as sufficient flexibility and high mechanical strength, application of graphene to a variety of products has been attempted (see Patent Documents 1 to 3). In addition, a technique for applying graphene to a lithium-ion secondary battery has been proposed (see Patent Document 4).